The present invention relates to a drive system having one or more synchronous motors driving a common load. In particular it relates to an improved electric inching control for synchronous motors driving grinding mills.
Frequently, a charge in a mill becomes frozen as a result of mill shut-down. Therefore, prior to starting the mill, it is common practice to "electrically inch" the mill by gradually rotating the rotors of the synchronous motors so that the mill is moved in steps which allow the mill charge to gradually move and tumble (i.e. to cascade or flow freely); thereby, preventing the lumped charge from falling instantaneously within the mill. In order to ensure that the mill can be inched properly, it is known to fully close the fluid actuated clutches, mechanically coupling the pinions driving the ring gear of the mill and the drive shafts of the synchronous motors. When low frequency power is applied from the inching bus, each pole of the field winding on the rotor (rotor pole) is positioned in alignment with and adjacent to a corresponding stator pole of opposite polarity on the stator winding. It is convenient to provide a rough approximation of low frequency power for the stator windings by using a direct current supply in combination with contactors or switches which apply the positive or negative voltages to predetermined ones of the three phase windings on each stator in a predetermined sequence.
Inching the mill electrically involves rotating the rotor so that each rotor pole, positioned adjacent its corresponding stator pole, is advanced to a position adjacent and in alignment with the next stator pole on the stator winding. The movement of each rotor pole of the two synchronous motor rotors is accomplished by energizing the stator windings with a low frequency alternating current and energizing the field windings of the rotors with direct current. The sequential application of the direct current voltage to the different phases of the three phase stator winding is preferably equivalent to a stator voltage of a frequency of the order of 1 cycle per second. Technical Information Bulletin GET-1722B entitled "Spotting Equipment for Synchronous Motors", E. A. E. Rich, published November 1966 by the General Electric Company, describes the use of two switches or contactors for each phase to connect the phase when required to either the positive or the negative line of a direct current supply, and thereby, by various connections, cause the rotor to rotate (see for example FIGS. 1 and 3 and related description).
A problem inherent in current electrical inching systems for mills presents itself when the rotors of the synchronous motors are slowly rotated, i.e. are stepped from one position to the next. During rotation of the rotors, each rotor pole overshoots when it advances to a position adjacent the next stator pole. The overshooting occurs because of the large momentum provided by the charged load, the mill and the rotors of the synchronous machines. Furthermore, after the rotors overshoot the next stator pole position, the magnetic field generated by the stator winding of each of the motors pulls each rotor pole back into alignment with the corresponding stator pole that has been overshot. Due to the momentum and the magnetic field, the rotor tends to oscillate. The disadvantage with the overshooting and oscillation of the rotor primarily resides in the destructive effect this vibrational movement has on the teeth of the ring gear and the pinions, which teeth mechanically couple the ring gear and the pinions. These teeth are subject to considerable adverse mechanical forces such as, for example, various strains and stresses during the oscillation of the rotor. The continued presence of such adverse forces on the teeth may result in breaking the teeth of either the pinions and/or the ring gear. As can be appreciated, it is expensive in terms of both material cost and mill down time to replace the gear.